1. Field of the Invention
The invention relates to a system for monitoring the rate of flow of packets on a network, and more specifically to a system for monitoring and controlling the rate of flow of cells into an ATM network.
2. Description of the Related Art
In recent years, communications services that uniformly treat various forms of communications of voice data, text data, video data, and so on have become essential. As a core technology for such communications services, ATM (Asynchronous Transfer Mode) has been researched and developed.
In ATM, information is transferred on a cell-by-cell basis, each cell consisting of a 48-octet payload and control information (including routing information) called a header. Upon receipt of a cell from a source terminal, an ATM network routes the cell to a destination terminal in accordance with routing information stored in its header.
In ATM, when a call connection is made, the transfer rate (the bandwidth that the call uses) in that connection is declared. The ATM network makes a decision of whether the connection can be established at the declared transfer rate by means of its CAC (Connection Admission Control) function. If the connection cannot be established, the call connection request is refused.
The source terminal normally outputs cells so as not to exceed a transfer rate declared for each connection. If the actual cell transfer rate in each connection is the declared transfer rate or less, cells will not be discarded in the ATM network. However, if cells are output at a rate exceeding the declared transfer rate in some connections, then the ATM network will be congested, resulting in discarding of cells.
In this case, it is not certain that in which connections cells are discarded. Therefore, it is also quite possible that cell discarding occurs even in connections that do not exceed the declared transfer rate.
In order to avoid such a problem, it is required to monitor the flow of cells into the ATM network at its input port and, if there is a connection having a cell flow rate that exceeds the declared flow rate, to discard cells that are transferred over that connection. In general, such monitoring is performed by a UPC (Usage Parameter Control) facility installed in an UNI (User Network Interface) at the input port of the ATM network. The UPC facility monitors the flow rate of cells for each connection and, when there is a connection having a cell flow rate in excess of the declared rate, discards nonconforming cells that are transferred over that connection or sets their CLP (Cell Loss Priority) bit to non-priority state so that their priority is decreased.
The use of the UPC facility to control the discard of nonconforming cells permits declared transfer rates to be observed in all the connections in the ATM network. With the system in which the CLP bit of nonconforming cells is set to non-priority state, when network congestion occurs, non-priority cells are discarded in the ATM network, and consequently cells associated with a connection having a cell flow rate over its declared rate will be discarded.
In an ATM network, a plurality of connections can be established on one physical link (or one virtual path). Suppose that two connections are established on one physical link and the cell transfer rate in the first connection is sufficiently low in comparison with its declared transfer rate. And when it is desired to transfer cells over the second connection at a transfer rate exceeding its declared transfer rate, it becomes possible to provide a cell transfer service at a transfer rate higher than the declared transfer rate by transferring part of cells to be transferred over the second connection over the first connection. According to such a service, the transfer rate of each connection can be increased without broadening the bandwidth of the physical link, permitting efficient utilization of ATM network resources.
However, in the conventional configuration which uses the UPC facility to discard nonconforming cells for each connection as described above, the transfer rate in each connection is not allowed to exceed its declared transfer rate and, even if a low-usage connection is present on the same physical link, another connection cannot utilize the bandwidth of that connection. For this reason, a request for efficient utilization of ATM network resources cannot be satisfied.
Note that the above-described problem arises in packet networks as well as ATM networks.